1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT) unit having an in-line type electron gun, a deflection yoke disposed on the color CRT so as to generate substantially uniform horizontal and vertical deflection magnetic fields, and a dynamic convergence yoke disposed near the front portion of the in-line type electron gun so as to apply a horizontal outward deflection force to two side electron beams passing therethrough, thus ensuring high resolution over the whole area of a phosphor screen thereof.
2. Description of the Prior Art
In general, a deflection yoke disposed on a color CRT having an in-line type electron gun generates a horizontal deflection magnetic field distorted in a pin-cushion shape and a vertical deflection magnetic field distorted in a barrel shape. Such a color CRT is capable of providing itself with a self convergence structure, thereby eliminating the necessity of a dynamic convergence circuit and allowing simplification of the circuit structure and reduction of power consumption.
However, since both the horizontal and vertical deflection magnetic fields are non-uniform, electron beams passing through these fields are distorted in their sectional shape as the deflection angle thereof becomes widened. This causes distortion of beam spots into a non-circular shape on a phosphor screen of the color CRT, especially on the peripheral area thereof, thereby deteriorating the resolution on the area.
In order to prevent the deterioration of resolution due to the above-described deflection distortion, Japanese Laid-Open Patent Publication No. 64-65753 proposes a system in which a deflection yoke generating substantially uniform horizontal and vertical deflection magnetic fields is disposed on a color CRT having an in-line type electron gun while misconvergence in the horizontal direction caused by the uniform deflection magnetic fields is corrected by a means of dynamic convergence.
Under the proposed system, as shown in FIG. 11 and FIG. 12, a color CRT 1 is provided with a top unit 3 for dynamic convergence at the front end of an in-line type electron gun 2. The top unit 3 includes in its cup-shaped body 4 made of non-magnetic metal, two pairs of magnetic members 5a and 5b and 6a and 6b, and a pair of magnetic shield members 7a and 7b. Dynamic convergence yokes 8a and 8b magnetically coupled with the pairs of magnetic members 5a and 5b and 6a and 6b respectively are disposed outside the tube. Each magnetic excitation coil of the yokes 8a and 8b receives a parabolic current which increases as the deflection angle of electron beams is widened, and, accordingly, side electron beams B and R are subjected to a horizontal outward deflection force. Reference numerals 9a, 9b and 9c denote three horizontally lined cathodes. The reference numerals 10, 11, 12, 13, 14 and 15 denote a control grid, an accelerating grid, a focusing grid, an anode, a phosphor screen and a deflection yoke which generates substantially uniform horizontal and vertical deflection magnetic fields, respectively.
The color CRT constructed in this way is advantageously effective to prevent the beam spots from being distorted on the peripheral area of the phosphor screen 14. However, the disadvantage is that a "dynamic focusing effect" is unavoidably caused; more specifically, as the deflection angle is widened so as to minimize the horizontal diameter of the beam spots, the parabolic dynamic voltage rises. If it is applied to a focusing electrode 12, the shapes of beam spots on the phosphor screen 14, especially those formed on the peripheral area thereof, are distorted, as shown in FIG. 13.
Referring to FIG. 14, the reason why this distortion is caused will be described:
When a dynamic current flows through the coils 16a and 16b of the yokes 8a and 8b respectively. a four-pole magnetic field is generated. This magnetic field generates uniform two-pole magnetic fields between two pairs of magnetic members 5a and 5b and 6a and 6b, respectively, as shown in FIG. 12. As a result, each of two side electron beams B and R receives a horizontal outward deflection force. However, in areas where such magnetic members are not disposed, that is, at both sides of the phosphor screen in the top unit 3 and toward the anode thereof, the four-pole magnetic fields leaking from the pair of yokes 8a and 8b directly affect three electron beams B, G and R. Thus, these electron beams are caused to deflect in the directions indicated by the large arrows in FIG. 15; more specifically, these beams are urged to diverge horizontally and to focus vertically, thereby prolonging beam spots horizontally as illustrated in FIG. 16.
As discussed above, the proposed color CRT system, regardless of the fact that it is provided with deflection yoke which generate substantially uniform horizontal and vertical deflection magnetic fields, does not satisfactorily improve resolution on the peripheral area of a phosphor screen.